Key Points at a Glance

Insolation

• Solar radiation received by Earth's surface; equatorial regions receive ~2× more than polar
• Varies with angle of incidence, day length, atmospheric path length, and albedo
• Arctic summer sees 24 hours daylight; oblique rays travel further through atmosphere
• Annual insolation at equator: ~1,370 W/m² (solar constant)

Albedo

• Reflectivity of a surface — fraction of incoming solar radiation reflected back
• Fresh snow: 80–90%; ice: 70–80%; desert sand: 35–40%; ocean: 6%; tropical forest: 12–15%
• Global average: ~30%; high albedo surfaces cool the climate
• Low albedo surfaces (dark ocean, forest) absorb more heat

Atmospheric Pressure Belts

• Equatorial Low (0°): Intense heating, rising air, low pressure, heavy rain
• Subtropical High (30° N/S): Subsiding air, dry, clear skies, deserts
• Subpolar Low (60° N/S): Meeting of cold polar and warm subtropical air
• Polar High (90°): Extremely cold, dense, descending air

Three-Cell Atmospheric Circulation

• Hadley Cell (0°–30°): Hot air rises at equator, moves poleward, cools and descends at 30° creating subtropical highs
• Ferrel Cell (30°–60°): Indirect cell; surface winds blow poleward
• Polar Cell (60°–90°): Cold air descends at poles, flows equatorward

Coriolis Effect

• Due to Earth's rotation, winds deflect right in Northern Hemisphere, left in Southern Hemisphere (Buys-Ballot's Law)
• Trade Winds blow from NE (NH) and SE (SH); Westerlies from SW (NH) and NW (SH)
• Cyclones rotate anticlockwise in NH and clockwise in SH

Global Wind Systems

• Trade Winds (0°–30°): NE Trades in NH, SE Trades in SH; most reliable winds on Earth
• Westerlies (30°–60°): Blow toward subpolar low; stronger in SH — "Roaring Forties," "Furious Fifties"
• Polar Easterlies (60°–90°): Cold, dry winds from polar high

Jet Streams

• Fast-flowing, narrow upper-air currents at 9–16 km altitude, speeds 120–400 km/h
• Subtropical Jet (30° N/S, ~12 km): Influences subtropical weather year-round
• Polar Front Jet (60° N/S, ~9 km): Controls mid-latitude weather systems
• Tropical Easterly Jet: Drives Indian Monsoon northward

Humidity

• Absolute humidity: Mass of water vapour per unit volume (g/m³)
• Relative Humidity (RH): Ratio of actual vapour to saturation vapour × 100%; measured by hygrometer/psychrometer
• Specific humidity: Mass of water vapour per mass of moist air (g/kg)
• Dew point: Temperature at which cooling air reaches 100% RH (saturation)

Types of Precipitation

• Convectional rain: Heated air rises vertically — equatorial/monsoonal regions; afternoon thunderstorms
• Orographic/Relief rain: Windward side of mountains — Western Ghats, Cherrapunji 11,430 mm/year
• Frontal/Cyclonic rain: Cold and warm air masses meet — temperate regions, moderate, prolonged
• Convective hail: Rapid updrafts freeze water droplets

World Climate Classification

• A (Tropical): Af, Am, Aw — all months above 18°C
• B (Dry): BWh (hot desert), BSh (semi-arid) — evaporation exceeds precipitation
• C (Temperate): Csa (Mediterranean), Cfb (Oceanic) — coldest month -3°C to 18°C
• D (Continental): Cold winters; E (Polar): ET (Tundra), EF (Ice cap)

El Niño–Southern Oscillation / ENSO

• El Niño: Anomalous warming of central/eastern Pacific (every 2–7 years); weakens trade winds
• Effects: Drought in Australia, India, SE Asia; floods in Peru/Ecuador; warmer global temperatures
• La Niña: Opposite — stronger trades, cooler Pacific; strengthens Indian Monsoon, floods in Australia
• SOI: Pressure difference between Tahiti and Darwin measures Southern Oscillation

Greenhouse Effect and Global Warming

• Natural greenhouse effect maintained by CO₂, water vapour, CH₄, N₂O; without it Earth avg = −18°C
• Enhanced greenhouse effect from fossil fuel burning — CO₂ at 425 ppm (2024) vs pre-industrial 280 ppm
• Global avg temperature risen 1.1°C above pre-industrial level (2024)